Mesh is gone after Bool Tool undo
BoolTool changes the way an object is displayed in the 3D window so that you can see the effect of the Boolean operation without the object getting in the way.  It looks like when you undid the BoolTool operation the object remained displayed as &quot;Bounds.&quot;
Here's how to check this:
In the properties editor, in the &quot;Object Properties&quot; tab there is a &quot;Viewport Display&quot; item.  Open it and it will show you a list of things that are displayed about the object:

The next to last item is &quot;Display As&quot;. This has a drop down menu:

If you leave it as &quot;Textured&quot;, then you get the normal behavior where the 3D menu for scene display decides whether to show the wire frame, solid, textured, or rendered view, via:

But if you set it to one of the other values, then it will display the object as no more that that form.
BoolTool does something fairly common in the Boolean based modeling workflow so as to avoid confusing you. Here's an example of a Boolean where I am using a cylinder to cut a hole in the default box:

Because the cylinder is still visible, you can't see the hole in the cube.  So BoolTool and other boolean based workflows like hardops or boxcutter set the cylinder to only display as wireframe or bounding box: